As a magnetic tape cartridge used for an external storage medium of data backup for computers, the magnetic tape cartridge of LTO (Linear Tape Open) standard has been known hitherto. The magnetic tape cartridge of this type has a case composed of the upper and lower half sections, in which a single reel wound with a magnetic tape is housed. The cartridge case possesses an opening for extracting the tape with a leader pin, and another opening for driving the reel, through which the cup-like hub of the reel is exposed, is made on the lower half section of the cartridge case.
A drive unit, into which the magnetic tape cartridge of this type is loaded, extracts the leader pin to wind out the magnetic tape through the opening of the cartridge case and performs recording or playback of the data. It also rewinds the magnetic tape around the reel by driving the cup-like hub through the opening of the lower half section and performs recording or playback of the data. The magnetic tape cartridge has a lock mechanism including a lock plate and a release pad in the small space in the cup-like hub of the reel. The lock plate is capable of locking the reel in order that the reel rotates only when it is loaded in the drive unit and does not rotate inadvertently when it is extracted from the drive unit, and also the slack of the magnetic tape while it is not in use is prevented. The release pad is able to release locking of the reel.
The lock plate is a disk-like member and has a first engaging portion capable of engaging or disengaging with a second engaging portion standing on the periphery of the inner surface (upper surface) of the base plate of the cup-like hub of the reel. Also it has a third engaging portion, which continuously engages with a fourth engaging portion projecting from the middle of the inner surface of the upper half section of the cartridge case. The lock plate will retain the reel lock position by engaging the first engaging portion with the second engaging portion by the force imposed by a compression coil spring provided between the lock plate and the upper half section.
The reel, on which the lock mechanism is mounted, will be described, referring to FIG. 23. It has been so arranged hitherto that an upper flange 115, which is a disk-like plate with a circular opening 115a in its center, is fixed to a flanged hub 110, which has a lower flange 112 integrally molded with a base plate 111a of a cup-like hub 111. A rib 115b to fit with the cup-like hub 111 is provided on the rim of the opening 115a of the upper flange 115 thoroughly along the inner cylindrical wall of the cup-like hub 111 in order to center the axis of the flanged hub 110 with that of the upper flange 115.
On the other hand, a release pad 120 is a plate member such as a rectangle, quadrilateral, star or propeller-like shape and disposed between the inner surface (upper surface) of the base plate 111a of the cup-like hub 111 of the reel and the lock plate (not shown). Lock releasing pins 121, which penetrate the base plate 111a of the cup-like hub 111, extend from the lower surfaces of the respective corners of the lock release pad 120 facing closely the inner surface of the cylindrical wall of the cup-like hub 111. When the magnetic tape cartridge is loaded into a drive unit and the cup-like hub 111 is rotationally driven by a reel drive mechanism (not shown) of the drive unit, each lock release pin 121 is forced to displace toward the lock plate (not shown). Accordingly the release pad 120 will press the lock plate (not shown) toward the upper half section against the force of a compression coil spring, thereby releasing the locked reel by disengaging the first engaging portion under the lock plate.
Since the rib 115b is provided around the periphery of the upper flange 115 which is melt-bonded to the opening of the cup-like hub 111 of the reel, the rib 115b overhangs the inner surface of the cylindrical wall of the cup-like hub 111. So, when the release pad 120 is mounted inside the cup-like hub 111 of the reel during the assembly of the magnetic tape cartridge, the release pad 120 is inserted slantwise into the cup-like hub 111 in order to avoid the interference with the inner circumference (the rib 115b) of the upper flange 115, and each lock release pin 121 of the release pad 120 is aligned with and inserted into each guide hole 122.
Each guide hole 122, which is made at the base plate of the cup-like hub 111, is positioned near the inner surface of the cylindrical wall of the cup-like hub 111 in accordance with the circumference of a circle required by LTO standard. Therefore, it is difficult to align each lock release pin 121 of the release pad 120, which is inserted slantwise into the cup-like hub 111, with each guide hole 122 securely, which will possibly lead to a defective mounting of the release pad 120 if a robot is introduced into the assembly. In this connection, it is inherently difficult to align each lock release pin 121 of the release pad 120 with each guide hole 122 securely, even if the release pad 120 can be inserted into the cup-like hub 111 horizontally when the inner circumference of the flange does not overhang the inner surface of the cylindrical wall (without rib 115b). The defective mounting of the release pad 120 will also possibly occur if the mounting is performed by an assembly robot.